This invention relates to improving the maintenance of high intensity discharge (HID) lamps over the lamps' lifetime. More specifically this invention provides methods for reducing the sputtering of cathode material during HID lamp starting.
The starting process of Metal Halide HID lamps consists of four phases: breakdown, glow, glow-to-arc transition, and thermionic arc. During each of these phases, high voltage and low pressure conditions promote the sputtering of electrode material by energetic ion bombardment at the cathode surface. The relative number of tungsten atoms sputtered in each of the above four starting phases is variable and is strongly dependent on the discharge conditions.
The effective number of atoms sputtered during the glow phase is relatively small. Similarly, only a small number of tungsten atoms are sputtered during breakdown formation, as this process is very fast and corresponding current is typically low.
The highest tungsten sputtering rate in the starting process occurs during the glow to arc transition where sputtering is enhanced under the high voltage and high current conditions required for this transition. Tungsten sputtering also occurs during the thermionic arc phase which is characterized by low gas pressure, high current, and voltage drop often above the sputtering threshold. The threshold ion energy necessary for tungsten sputtering is approximately 40 eV. The glow to arc transition occurs within a fraction of a second, and is much shorter than the thermionic arc phase which lasts typically 40 seconds in the 100 W Metalarc lamp. Therefore, although the rate of sputtering during the thermionic arc period is lower than the rate at the glow to arc transition, the length of the thermionic arc phase enables the relative number of atoms sputtered during the thermionic arc phase to exceed that of all other starting phases combined. In lamps where this is the case, tungsten atoms sputtered during the thermionic arc phase are most likely the dominant cause of wall blackening. In the normal operation of 100 W Metalarc lamps, the thermionic arc phase current is approximately 1 A RMS with voltage drops of approximately 45 V RMS. This corresponds to a peak current of 1.5 A and a peak voltage of 65 V.
During the thermionic arc phase the lamp heats up resulting in the vaporization of mercury condensate and metal halides causing an increase in gas pressure. The high pressure during the final stabilized arc discharge minimizes the number of sputtered atoms capable of diffusing through the discharge to the arc tube wall. Most of the atoms sputtered under high pressure conditions diffuse back to the cathode.
Thus, by decreasing the sputtering during the thermionic arc phase of HID lamp starting, the overall lamp wall blackening is decreased resulting in a lamp with improved maintenance. The present invention accomplishes this decrease in sputtering during the thermionic arc phase by increasing the open circuit voltage or decreasing ballast impedance and thereby enhancing the current flowing through the lamp during this phase.